drm/i915: Only apply a rmw mmio update if the value changes

If we try to clear, or even set, a bit in the register that doesn't
change the register state; skip the write. There's a slight danger in
that the register acts as a latch-on-write, but I do not think we use a
rmw cycle with any such latch registers.

Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190917080029.27632-1-chris@chris-wilson.co.uk